There have been a number of materials studied to initiate bone repair and/or to restore or replace missing bone. These studies have been undertaken in an effort to address the problem of stimulating formation of bone at specific sites.
Among the approaches used to address this problem is a conformational method in which an implant material, usually made of metal, ceramic or other inorganic material in a configuration intended to mimic the form of the missing bone, is inserted into the site in which bone replacement is required. With the technique, there is a risk that the host will reject the implant material or that the implant will fail to become integrated with normal skeletal tissue. Some ceramic materials such as ceramic tricalcium phosphate, although acceptably biocompatible with the host and bone, when used as an implant, appear to lack sufficient mechanical properties of bone for general utility. As a result, the bone does not consistently grow into, and become incorporated, within the implant.
Metallic and plastic implants are widely used but are limited in that their mechanical properties do not match that of bone, and the wear debris associated with these implants is known to cause osteolysis. Newer materials (titanates, silicates) can mimic the porosity of bone, and hence allow cellular incorporation, but the implants persist in the body. Calcium phosphate ceramics can also mimic bone porosity, and some of these are resorbable; but their reported success is limited. Calcium phosphate implants which mimic bone have been used in conjunction with proteins and growth factors, but these proteins are degraded more rapidly than the implant, in many cases before cells have penetrated the implant. Collagen implants persist in the tissues, and support the incorporation of cell binding factors; however these factors are rapidly degraded, and may be altered by sterilization.
Another approach, referred to as osteoconduction, involves substituting the missing bone tissue with a matrix which functions as a support into which the new bone growth can occur. The matrix attracts the cells committed to an osteogenic pathway, and the new bone grows in and through the matrix. Allogeneic (non-host) bone grafts are used for this method, however there is a high failure rate. Even when the allogeneic bone grafts are accepted by the host, healing periods for consolidation and capacity for mechanical stress are of comparatively long duration compared to autogeneic (host) bone grafting. The use of allogeneic bone also presents the issue of transmissible viral agents.
A third method for initiating bone repair involves the process known as osteoinduction, which occurs when a material induces the growth of new bone from the host's undifferentiated cells or tissues, usually around a temporary matrix. A number of compounds are disclosed as having such a capacity. See for example, U.S. Pat. No. 4,440,750 to Glowacki, U.S. Pat. Nos. 4,294,753 and 4,455,256 to Urist, and U.S. Pat. Nos. 4,434,094 and 4,627,982 to Seyedin et al. The most effective of these compounds appear to be proteins which stimulate osteogenesis. However, when synthesized from natural sources they are present in extremely low concentrations and require large amounts of starting material to obtain even a minute amount of material for experimentation. The availability of such proteins by recombinant methods may eventually make the use of such proteins per se of more practical value. However, such proteins will still need to be delivered to the desired site in an appropriate matrix.
Acidic phospholipids form unique complexes when incubated in the presence of inorganic phosphate and calcium (Boskey and Posner, Calcif. Tissue Res. 1976, 19:273-283; Boskey and Posner, Calcified Tissues Int. 1982, 34:s1-s7; Cotmore et al., Science 1971, 172:1339-41; Goldberg and Boskey, Prog. Histochem. Cytochem. 1996, 31(2):1-187). These complexes exist in mineralizing tissues as part of the “nucleational core” of extracellular matrix vesicles, the site of initial mineral deposition in cartilage, mantle dentin, and in newly forming bone (Anderson, Clin. Orthop. 1995, 314:266-80). They are also found associated with dystrophic apatite deposition, and their in situ formation is regulated by the same factors that promote bone formation (Tintut et al., Curr. Opin. Lipidol. 2001, 12:555-60; Boyan et al., Steroids 2001, 66:363-74). These complexes induce hydroxyapatite formation in vitro in the absence and presence of cells, and when implanted in vivo. Both the acidic phospholipids themselves (Ennever et al., Cytobios. 1984, 39:151-7) and the acidic phospholipid complexes bind with high affinity to collagen, and when bound are still able to induce mineral deposition (Boskey, J. Phys. Chem. 1989, 93:1628-1633).
Collagens are a class of naturally occurring biomaterials suitable for use in bone regeneration. A major protein constituent of connective tissue, collagen has been widely used in various medical and surgical applications such as for surgical prostheses and graft fabrication. In addition, collagen-based matrices have been used in bone grafting. Type I collagen has good cell adhesive properties, in particular, for bone forming osteoblast cells. Collagen has the capacity to serve both as an active or inert scaffold material for growth. Thus, compositions containing collagen and various forms of calcium phosphate directed to healing and bone growth have been disclosed. For example, U.S. Pat. No. 4,780,450 to Sauk et al. discloses a composition for bone repair comprising particulate polycrystalline calcium phosphate ceramic, a phosphophoryn calcium salt and a type I collagen in a weight ratio of 775-15:3-0.1:1. The ceramic particles are disclosed as being dense hydroxyapatite about 1 to 10 microns in diameter or larger dense hydroxyapatite ceramic particles greater than about 100 microns in diameter.
Similarly, PCT Application WO 94/15653 to Ammann et al. discloses formulations comprising tricalcium phosphate (TCP), TGF-β and collagen. The TCP is disclosed as being a delivery vehicle for the TGF-β such that the TCP is of the particle size greater than 5 microns and preferably greater than about 75 microns. However, the use of solubilized collagen and collagen sheets and blocks for repair of osseous defects in bone and full thickness defects in cartilage have been troubled by the rapid degradation or loss of growth factors and peptides linked to these materials to stimulate osteointegration, and by the loss of these peptides during sterilization. The in vitro nucleation of apatite by the acidic phospholipid complex and collagen composite has been demonstrated in the absence of cells (Boskey, J. Phys. Chem. 1989, 93:1628-1633).
To date, attempts have been made to add bone-inducing substances (i.e., growth factors, peptides) to collagen to promote osteoinduction. However, these attempts have initiated the osteoinduction process with bone (demineralized) rather than collagen. In addition, sterilization techniques may compromise the structural and biochemical properties of the growth factor complexes. Therefore, there exists a need for an osteoinductive material which remains stable after sterilization and advantageously provides reliable bone growth.